Rudy Nappee has just finished what he calls the hardest week in his Google Summer of Code project - work on the PDF output methods:

Here was the hardest GSoC week ! I wrote a big part of the PDF output and both the themes "phppdf" (each PHP manual part in a file) and "phpbigpdf" (a big PDF file with all the sections).

Rudy's project for the GSoC is to create a Docbook renderer that can take in a standard Docbook file and output it into multiple formats. This past week saw the development of the PDF output method but he's also created methods for making a CHM (Windows help file) and a manpage output format. You can keep up with his latest progress on his blog.